A control system typically comprises one or more physical system components under some form of automated control that cooperate to achieve a set of common objectives. The control system may be designed to reliably control the physical system components in the presence of external disturbances, variations among physical components due to manufacturing tolerances, and changes in commanded input values for controlled output values, such as a cement mixture density, for example. The control system may also be designed to remain stable and avoid oscillations within a range of specific operating conditions.
In a well bore environment, a control system may be used when mixing materials to achieve a desired mixture output. For example, when drilling an oil or gas well, it is common to install a tubular casing into the well bore and cement the casing in place against the well bore wall. A cement mixing system that supports well bore servicing operations, such as cementing casing into a well bore, may be designed with a control system configured to provide a desired volumetric flow rate of mixed cement having a desired density. In particular, the cement mixing control system may control valves that allow the in-flow of dry cement material and water to obtain the desired cement mixture density and desired cement mixture volumetric flow rate. The control system may operate, for example, by monitoring the cement mixture flow rate and density, and by regulating an in-flow water valve and an in-flow dry cement material valve. However, because such systems conventionally control the output parameters, such as cement mixture flow rate and density, dependently, these systems tend to have long lag times in the response of one valve to changes in the position of the other valve. This can lead to unacceptable oscillations in the monitored parameters, and difficulty in stabilizing the system. Therefore, to make the system more stable, it would be desirable to control output parameters, such as a mixture flow rate and a mixture density, for example, independently of each other. Accordingly, a need exists for a mixing control system with multiple inputs that decouples the effects of changes in the commanded outputs.